This invention relates to the art of shift registers and more particularly, to serial access, thin-film magnetic memories using domain walls for the storage and propagation of binary information and to processes for making and using these devices. Prior thin-film shift register art discloses serial access memories with storage and propagation of reversal domains, e.g., U.S. Pat. No. 3,846,770, and inversion of Neel walls with storage and propagation of Bloch lines, crossties, and inverted Neel walls along a domain wall, e.g., U.S. Pat. No. 3,868,659. The development of thin magnetic film shift registers with domain wall storage and propagation is quite recent, and for the benefit of those who may not be familiar with this art, a brief review follows.
In a serial access memory on a thin magnetic film such as a 320 A thick layer of 80%-20% Ni-Fe composition, a plurality of magnetic domains are placed on the thin-film layer by applying electrical currents through wires placed over the layer. Adjacent domains define a common boundary known as a domain wall in which digital information in various conventions may be stored and propagated. Only the data moves in the domain walls with recording and detecting heads, as well as the thin permalloy film layer, remaining stationary. The digital information is read into the memory by placing a fine wire above and parallel to the domain wall and applying a current pulse of proper polarity to invert the Neel wall, form Bloch lines and crossties. Digital information stored in the domain wall is propagated by varying the field produced by conductors located above the domain wall, causing movement of the Bloch lines and relocation of the crossties along the domain wall.
The margins of thin magnetic film memories as taught in the prior art, appear uniformly smooth. Consistency in the travel of the digital information during its propagation necessitates a boundary definition of the individual memory cells in which the crossties and Bloch lines rest between steps in their journey along the domain wall. In one device, a thin wire called a "meander line" through which direct currents of alternating polarity run, was placed in a 45 degree zig-zag configuration above the domain wall. At any particular instant this creates adjoining local magnetic fields of opposite polarity, thereby discouraging a crosstie or Bloch line from progressing more than one cell during each propagation sequence. In another device, the drive line is arranged in series of coplanar straight segments coupled at right angles to form alternately reversed crenelations. The drive current defines adjoining cells by creating local magnetic fields in the plane of the thin-film surface that are oriented antiparallel to each other and normal to the domain wall.
As taught by the prior art, domain walls are placed on the thin-films by one of several techniques such as depositing the film in the presence of localized magnetic fields, annealing the thin-film in the presence of localized magnetic fields, or placing the domain walls at a desired location and then annealing the film in the presence of a hard axis magnetic field. In order to correctly locate the detector, the domain walls must be placed very accurately. The use of a thermal annealing technique for example, allows placement of domain walls with an accuracy of .+-.1 micron over a 2.5 centimeter length. Alignment of the detector with the domain walls necessitates marking the domain walls. Typically, holes are burned into the thin-film by discharging a capacitor through a tungsten needle held over the domain walls under Kerr effect observation. The holes are mapped under higher magnification and fiducial marks are placed on the substance through a mask aligner, thus providing for the alignment of subsequent masks. Although viable for domain wall placement as taught by the prior art, it is apparent that the techniques are tedious, requiring great manual patience. The lack of readily distinguishable features on the surface of the thin-film means that these devices are not especially susceptible to mass production.